An ion source is a device that ionizes gas molecules and then focuses, accelerates, and emits them as a narrow beam. This beam is then used for various technical and technological purposes such as cleaning, activation, polishing, thin-film coating, or etching.
An example of wide-aperture type ion sources intended for treating objects with large surface area is the so-called Kaufman ion source, also known as a Kaufman ion engine or an electron-bombardment ion source described in U.S. Pat. No. 4,684,848 issued to H. R. Kaufman in 1987.
This ion source consists of a discharge chamber, in which plasma is formed, and an ion-optical system, which generates and accelerates an ion beam to an appropriate level of energy. A working medium is supplied to the discharge chamber, which contains a hot cathode that functions as a source of electrons and is used for igniting and maintaining a gas discharge. The plasma, which is formed in the discharge chamber, acts as an emitter of ions and creates, in the vicinity of the ion-optical system, an ion-emitting surface. As a result, the ion-optical system extracts ions from the aforementioned ion-emitting surface, accelerates them to a required energy level, and forms an ion beam of a required configuration. Typically, aforementioned ion sources utilize two-grid or three-grid ion-optical systems.
A disadvantage of such a device is that it does not provide a uniform ion-emitting surface, especially when the ion-emitting surface is large. Another disadvantage is that it requires the use of ion accelerating grids which contaminate the ion beam, and hence the surface of the object, as a result of sputtering of the material from the surface of the grid-like electrode. Another disadvantage of the Kaufman ion sources is that the ion beams emitted from such sources are still of insufficient intensity.
Attempts have been made to provide ion sources with ion beams of higher intensity by holding the electrons in a closed space between a cathode and an anode where the electrons could be held. For example, U.S. Pat. No. 4,122,347 issued in 1978 to Kovalsky et al. describes an ion source with a closed-loop trajectory of electrons for ion-beam etching and deposition of thin films, wherein the ions are taken from the boundaries of a plasma formed in a gas-discharge chamber with a hot cathode. The ion beam is intensified by a flow of electrons which are held in crossed electrical and magnetic fields within the accelerating space and which compensate for the positive spatial charge of the ion beam.
A disadvantage of devices of such type is that they do not allow formation of ion beams of chemically-active substances for ion beams capable of treating large surface areas. Other disadvantages of the aforementioned devices are short service life and high non-uniformity of ion beams.
Russian Patent No. 2,030,807 issued in 1995 to M. Parfenyonok, et al. describes an ion source that comprises a magnetoconductive housing used as a cathode having an ion-emitting slit, an anode arranged in the housing symmetrically with respect to the emitting slit, a magnetomotance source, a working gas supply system, and a source of electric power supply.
For better understanding the construction and principle of operation of an ion-beam source with a closed-loop ion-beam emitting slit and electrons drifting in crossed electric and magnetic fields, to which the present invention pertains, it would be expedient to describe the construction and operation of such a source in more detail.
FIGS. 1 and 2 schematically illustrate the aforementioned known ion source with a circular ion-beam emitting slit. More specifically, FIG. 1 is a sectional side view of an ion-beam source with a circular ion-beam emitting slit, and FIG. 2 is a sectional plan view along line II--II of FIG. 1.
The ion source of FIGS. 1 and 2 has a hollow cylindrical housing 40 made of a magnetoconductive material such as Armco steel (a type of mild steel), which is used as a cathode. Cathode 40 has a cylindrical side wall 42, a closed flat bottom 44 and a flat top side 46 with a circular ion emitting slit 52.
A working gas supply hole 53 is formed in flat bottom 44. Flat top side 46 functions as an accelerating electrode. Placed inside the interior of hollow cylindrical housing 40 between bottom 44 and top side 46 is a magnetic system in the form of a cylindrical permanent magnet 66 with poles N and S of opposite polarity. An N-pole faces flat top side 46 and S-pole faces bottom side 44 of the ion source. The purpose of a magnetic system 66 with a closed magnetic circuit formed by parts 66, 40, 42, and 44 is to induce a magnetic field in ion emitting slit 52. It is understood that this magnetic system is shown only as an example and that it can be formed in a manner described, e.g., in aforementioned U.S. Pat. No. 4,122,347. A circular annular-shaped anode 54, that is connected to a positive pole 56a of an electric power source 56, is arranged in the interior of housing 40 around magnet 66 and concentric thereto. Anode 54 is fixed inside housing 40 by means of a ring 48 made of a non-magnetic dielectric material such as ceramic. Anode 54 has a central opening 55 in which aforementioned permanent magnet 66 is installed with a gap between the outer surface of the magnet and the inner wall of opening 55. A negative pole 56b of electric power source is connected to housing 40, which is grounded at GR.
Located above housing 40 of the ion source of FIGS. 1 and 2 is a sealed vacuum chamber 57 which has an evacuation port 59 connected to a source of vacuum (not shown). An object OB to be treated is supported within chamber 57 above ion emitting slit 52, e.g., by connecting it to an insulator block 61 rigidly attached to the housing of vacuum chamber 57 by a bolt 63 but so that object OB remains electrically and magnetically isolated from the housing of vacuum chamber 57. However, object OB is electrically connected via a line 56c to negative pole 56b of power source 56. Since the interior of housing 40 communicates with the interior of vacuum chamber 57, all lines that electrically connect power source 56 with anode 54 and object OB should pass into the interior of housing 40 and vacuum chamber 57 via conventional commercially-produced electrical feedthrough devices which allow electrical connections with parts and mechanisms of sealed chambers without violation of their sealing conditions. In FIG. 1, these feedthrough devices are shown schematically and designated by reference numerals 40a and 57a. Reference numeral 57b designates a seal for sealing connection of vacuum chamber 57 to housing 40.
The known ion source of the type shown in FIGS. 1 and 2 is intended for the formation of a unilaterally directed tubular ion beam. The source of FIGS. 1 and 2 forms a tubular ion beam IB emitted in the direction of arrow A and operates as follows.
Vacuum chamber 57 is evacuated, and a working gas is fed into the interior of housing 40 of the ion source. A magnetic field is generated by magnet 66 in the accelerating gap between anode 54 and cathode 40, whereby electrons begin to drift in a closed path within the crossed electrical and magnetic fields. Plasma 58 is formed between anode 54 and cathode 40. When the working gas is passed through the ionization gap, tubular ion beam IB, which is propagated in the axial direction of the ion source shown by an arrow A, is formed in the area of an ion-emitting slit 52 and in an accelerating gap 52a between anode 54 and cathode 40.
The above description of the electron drift is simplified to ease understanding of the principle of the invention. In reality, the phenomenon of generation of ions in the ion source with a closed-loop drift of electrons in crossed electric and magnetic fields is of a more complicated nature and consists in the following.
When, at starting the ion source, a voltage between anode 54 and cathode 40 reaches a predetermined level, a gas discharge occurs in anode-cathode gap 52a. As a result, the electrons, which have been generated as a result of ionization, begin to migrate towards anode 54 under the effect of collisions and oscillations. After being accelerated by the electric field, the ions pass through ion-emitting slit 52 and are emitted from the ion source. Inside the ion-emitting slit, the crossed electric and magnetic fields force the electrons to move along closed cycloid trajectories. This phenomenon is known as "magnetization" of electrons. The magnetized electrons remain drifting in a closed space between two parts of the cathode, i.e., between those facing parts of cathode 40 which form ion-emitting slit 52. The radius of the cycloid is, in fact, the so-called doubled Larmor radius R.sub.L which is represented by the following formula:
R.sub.L =m.sub.e V/.vertline.e.vertline.B,
where m.sub.e is a mass of the electron, B is the strength of the magnetic field inside the slit, V is a velocity of the electrons in the direction perpendicular to the direction of the magnetic field, and .vertline.e.vertline. is the charge of the electron.
It is required that the height of the electron drifting space in the ion-emission direction be much greater than the aforementioned Larmor radius. This means that a part of the ionization area penetrates into ion-emitting slit 52 where electrons can be maintained in a drifting state over a long period of time. In other words, a spatial charge of high density is formed in ion-emitting slit 52.
When a working medium, such as argon which has neutral molecules, is injected into the slit, the molecules are ionized by the electrons present in this slit and are accelerated by the electric field. As a result, the thus formed ions are emitted from the slit towards the object. Since the spatial charge has high density, an ion beam of high density is formed. This beam can be converged or diverged by known technique for specific applications.
Thus, the electrons do not drift in a plane, but rather along cycloid trajectories across ion-emitting slit 52. However, for the sake of convenience of description, here and hereinafter such expression as "electron drifting plane" or "drifting in the plane of ion-beam propagation" will be used.
The diameter of the tubular ion beam formed by means of such an ion source may reach 500 mm and more.
The ion source of the type shown in FIG. 1 is not limited to a cylindrical configuration and may have an elliptical or an oval-shaped cross section as shown in FIG. 3. FIG. 3 is a cross-sectional view of the ion-beam source along line III--III of FIG. 1. In FIG. 3 the parts of the ion beam source that correspond to similar parts of the previous embodiment are designated by the same reference numerals with an addition of subscript OV. Structurally, this ion source is the same as the one shown in FIG. 1 with the exception that a cathode 40.sub.ov, anode 54.sub.ov, a magnet 66.sub.ov, and hence an emitting slit (not shown in FIG. 3), have an oval-shaped configuration. As a result, a belt-like ion beam having a width of up to 1400 mm can be formed. Such an ion beam source is suitable for treating large-surface objects when these objects are passed over ion beam IB emitted through emitting slit 52.
With 1 to 3 kV on the anode and various working gases, this source makes it possible to obtain ion beams with currents of 0.5 to 1 A. In this case, an average ion energy is within 400 to 1500 eV, and nonuniformity of treatment over the entire width of a 1400 mm-wide object does not exceed .+-.5%.
A disadvantage of the device described above is that, in treating objects of large surface areas, it does not provide uniformity in distribution of ion current density over the surface of an object being treated. Another disadvantage is that the distribution pattern of the ion-current density on the object surface cannot be controlled or adjusted. An increase in the intensity of the ion-beam current densities by increasing the area of the ion-emitting surface is limited.
An attempt has been made to improve uniformity of the current density distribution on the surface of the object by proving a multiple-cell ion-beam sources. One such device is described in USSR Author's Certificate No. 865043. As shown in FIG. 4, which is an elevational sectional view of the ion-beam source 100 of the aforementioned type, the device is made in the form of a multiple-cell source having two cathode plates 102 and 104 which function as magnetic poles. An anode plate 106 with openings is placed between cathode plates 102 and 106. Cathode plate 104 has rods 108a, 108b, 108c, which extend from cathode plate 104 to second cathode plate 102. Second cathode plate 102 has openings 110a, 110b, 110c coaxial with respective rods 108a, 108b, 108c and with openings in anode plate 106. The anode-cathode assembly is supported by a cup-shaped housing 112 of a magnetoconductive material, which contains an electromagnetic coil 114 for generating the aforementioned magnetic field in a anode-cathode space of ion source 100. In a conventional manner, entire ion source 100 is placed into a sealed vacuum chamber 118. A working medium is supplied to vacuum chamber via a working medium supply channel 116.
Rods 108a, 108b, 108c can be made of a magnetic or nonmagnetic material.
Thus, each opening 110a, 110b, 110c in cathode plate 102 and a respective coaxial rod 108a, 108b, 108c of the device form an individual ion-beam source of the type described above, i.e., of the type disclosed in Russian Patent No. 2,030,807. More specifically, the end of each rod and the adjacent opening in cathode plate 102 form a closed-loop ion-beam emitting slit, so that all rods and the openings in the second cathode plate form a plurality of such slits. In the context of the present invention, a combination of one rod with a respective opening will be referred to as a "cell", and the ion-beam source of this type will be called a "multiple-cell type ion-beam source". Cathode plates 102 and 104 are electrically isolated from anode plate 106 and grounded via a conductor 122. Anode plate 106 is connected to a source of a positive potential (not shown) via a conductor 124.
In operation, a working medium is supplied through channel 116 to the accelerating and ion-generating space between anode plate 106 and cathode plates 102, 104, and a potential difference is developed between the cathode plates and the anode plate. This generates crossed electric and magnetic fields in the aforementioned ion-emitting slits. These fields hold drifting electrons which ionize the working medium and compensate for the spatial charge of the ion beams IB1, IB2, IB3, which are emitted toward an object OB1 via openings in the second cathode plate. Object OB1 is fixed inside vacuum chamber 118.
Although the multiple-channel ion-beam source of the type described above to some extent improves uniformity of ion-current density distribution on the surface of an object being treated, it has a limitation with regard to the dimensions of the cathode plate for treating objects of large surface area.
More specifically, as shown in FIG. 5A, which is a fragmental top view illustrating lines of magnetic fields in adjacent cells, the magnetic flux generated by electromagnetic coil 114 (FIG. 4) in individual cells decreases towards the periphery of cathode plate 102 in proportion to R, if cathode plate 102 is round (where R is a current radius from the center to the periphery). This is because all the cells have magnetic poles of the same sign on the side of plate 102, so that the resulting magnetic flux is increased and its intensity is accumulated towards the cathode plate center. As a result, the material of the cathode, which normally is a mild steel such as Armco steel, is magnetically saturated, so that intensity of magnetic fields in the central cells is decreased. This results in nonuniformities in the distribution of the ion-current densities over the surface of the object being treated. Therefore an increase in the diameter or in the overall dimensions of the cathode plates is limited, since as the greater the cathode diameter, the greater is nonuniformities in the distribution of the magnetic flux over the cathode plate.
Another disadvantage of the multiple-cell ion-beam source of the aforementioned type is that it does not allow control or adjustment in the distribution of the ion current-density over the surface of the object.